Parathyroid hormone (PTH) is a well known skeletal mediator and is effective for building bone in situations of metabolic bone disease and localized osseous defects. Despite its clinical efficacy, its mechanisms of action are complex and have eluded bone biologists for decades. The anabolic actions of PTH are not simple activation of cells that form bone, but likely require accessory cells in an intricate temporal and spatial organization. Preliminary data from the project laboratory strongly support the ability of PTH to stimulate osteoblastic cell proliferation, but in an indirect manner through another cell type in the bone marrow microenvironment. Data is emerging of PTH actions to expand cells of the hematopoietic compartment in the bone marrow which may function in the impact of PTH in bone. The overall hypothesis is that PTH is anabolic in bone via its ability to increase osteoblast proliferation indirectly through modulation of cell/s of the hematopoietic lineage. Three specific aims will elucidate the cellular targets of PTH action in its impact on hematopoietic cells and also link the angiogenic potential of hematopoietic cells with the osteoblastic phenotype in the anabolic actions of PTH in bone. The first aim will identify and verify the hematopoietic cell/s that expand in response to PTH. The second aim will elucidate the mechanistic aspects of the increase in hematopoietic cell numbers in response to PTH as they depend on proliferative or anti-apoptotic cell signaling events. The third aim will determine if the increase in hematopoietic cell populations translates into increased osteoblast proliferation leading to bone formation and/or increased vascular support in bone in vivo. These studies will utilize novel animal models, solid cell signaling biochemical assays, and cell sorting strategies to address these aims. These studies will better clarify the cellular targets of PTH action in bone and extend our existing knowledge of key cellular regulators in bone to include cells of the hematopoietic lineage in the bone marrow microenvironment. The compelling impact of this work is in the application of PTH toward tissue regeneration such as in states of osseous defects due to development, trauma, and inflammatory diseases. It is likely that information garnered from these models will also be important for clarifying the mechanisms of PTH therapeutic action in the treatment of metabolic bone disease such as osteoporosis and in states of dysregulated PTH or PTHrP such as hyperparathyroidism and skeletal metastasis. PUBLIC HEALTH RELEVANCE: Parathyroid hormone (PTH) is a normal protein made by the body, and has also been developed as a drug for the treatment of bone disorders. The mechanisms of how PTH works are complex and have eluded scientists for many years. This project will identify the cells that are targets for PTH action in the bone marrow and elucidate the pathways of its action to build bone. Information garnered from these studies will be important for optimizing treatment of metabolic bone diseases such as osteoporosis, hyperparathyroidism and skeletal metastasis, and in designing strategies to regenerate bone in localized skeletal defects.